PROJECT ABSTRACT Most youth who experience significant trauma-related symptoms do not receive indicated services and, when they do, the services received are not evidence-based. The education sector is responsible for 70-80% of all mental health services delivered to youth, but there are (1) a limited menu of evidence-based treatments (EBTs) for trauma available for use in schools and (2) a need to enhance the uptake and use of EBT in schools and other community settings for youth exposed to trauma. Trauma-focused cognitive behavior therapy (TF-CBT) is an efficacious treatment for trauma-exposed youth, but there are no studies that have evaluated the effectiveness of TF-CBT when delivered in schools. Additionally, significant implementation barriers limit the successful adoption, delivery with fidelity, and sustainment of TF-CBT; especially individual clinician variables, such as beliefs and attitudes surrounding core components of the intervention (e.g., exposure to feared stimuli). These two gaps reveal the need for research that simultaneously examines effectiveness and implementation of TF-CBT when delivered in schools. Individual provider behavior change is a critical component of successful implementation of any EBT. Even when factors such as evidence-informed policy, supportive leadership, and effective training are in place, implementation ultimately rests with providers? motivation and commitment to change. The Beliefs and Attitudes for Successful Implementation in Schools (BASIS) intervention is a facilitated, group-based blended implementation strategy. BASIS is brief, pragmatic, and is designed to augment standard EBP training and consultation procedures. It targets theoretically-derived individual-level determinants associated with behavior change using motivational and volitional behavior change strategies. BASIS has been rated as feasible and appropriate by SBMH clinicians, and found to increase clinicians? attitudes, perceptions of social norms, and implementation intentions to use EBT in a small (n = 24) randomized trial (R21MH108714). Considering the need to examine the effectiveness of TF-CBT in the context of schools and the importance of individual-level strategies that augment EBT training and consultation, the current proposal is a hybrid type 2 effectiveness-implementation trial with 120 SBMH clinicians and 480 students using mixed methods. This project is designed to simultaneously test the effectiveness of TF-CBT in schools and the impact of BASIS on proximal mechanisms and TF-CBT implementation outcomes. Specifically, the proposal aims to experimentally evaluate TF-CBT in schools versus treatment as usual, including intervention mechanisms of change, mental health outcomes, and TF-CBT cost-effectiveness (Aim 1); and experimentally evaluate BASIS versus and attention control, including implementation mechanisms of change, TF-CBT implementation outcomes, and BASIS cost-effectiveness (Aim 2). TF-CBT and BASIS represent potentially robust and generalizable strategies for greatly enhancing the accessibility and quality of services for trauma-exposed youth.